After some experimentation, the limitation is obvious. I usually want to complete the entire filename including the path but I can’t remember what the path begins with.

The next thing I thought about was removing *Messages* and *Buffer List* from dabbrev-ignored-buffer-names. Unfortunately, I still need to complete from the beginning of the path but this is generally useful so I’m going to keep it. Really, I want to be able to select any part of the filename and complete from that. Sounds like a job for ido.